Principal Investigator/Program Director (Last, first, middle): McMahon, Steven, B Overexpression of MYC is among the most common genetic events in human cancer. Current models suggest that high levels of MYC, which is a sequence-specific transcription factor, regulate a transcription network that ultimately leads to malignant transformation. Despite intensive study, the essential genes that lie downstream of MYC, and the mechanism by which MYC regulates these genes, remains only partially understood. We have performed an expression profiling screen aimed selectively at the identification of MYC targets that are critical to the transformation process. Of the 629 genes previously reported as MYC targets, only 6 have documented roles in MYC mediated transformation. Our screen identified 20 novel MYC targets and validation studies are beginning to confirm their importance in MYC driven tumorigenesis. We have recently reported that shRNA mediated depletion of one of the novel targets, MTA1, blocks the ability of MYC to induce growth of cells in soft agar. Additional targets from the screen (POLRMT and CD30), have been thoroughly characterized at the molecular level. The first Aim of the new funding cycle is the characterization of the biological role these three targets play during MYC mediated transformation. Based on the success of our screen for essential, MYC activated targets, we propose in the second Aim to perform the first screen focused specifically on the identification of genes repressed by MYC. A great deal of evidence suggests that repressed targets are of particular importance during MYC induced transformation. As part of this Aim, we will define the co-repressor(s) that function with MYC to mediate transcriptional silencing. These studies will combine biochemical, molecular and genetic techniques and should ultimately provide a deeper understanding of the pathway regulated by MYC in human cancer. Project Description Page 6